In many cases where modern efficiency requirements are to be fulfilled, synchronous rectifiers “SR” need to be used instead of ordinary diode or thyristor rectifiers. In synchronous rectifiers, the operation of a rectification diode or thyristor is mimicked with a controllable rectification switch whose voltage-drop in the conducting state is smaller than that of a diode or a thyristor. The controllable rectification switch can be for example a metal oxide semiconductor field effect transistor “MOSFET”.
A synchronous rectifier can be for example a part of a secondary side of a switched mode power supply “SMPS”. In many traditional switched mode power supply topologies such as e.g. the flyback topology, the operation of the secondary side is in phase with the operation of the primary side, which makes it relatively easy to implement the control of the synchronous rectifier with the aid of control signals of the primary side. In conjunction with resonant converters, the situation is, however, more complicated because a resonant converter comprises a resonance circuit which is supplied by switched mode voltage controlled by the primary switches and which is connected to the primary winding of the transformer of the resonant converter. The absolute value and the angle of the impedance of the resonance circuit are frequency dependent. Hence, the amplitude of the current supplied to the primary winding of the transformer can be controlled by altering the frequency of the fundamental component of the switched mode voltage. The amount of power transferred to the output of the resonant converter can thus be controlled by altering the above-mentioned frequency. The frequency dependent impedance of the resonance circuit causes a frequency dependent phase-shift between the operation of the primary side and the operation of the secondary side of the resonant converter, where the phase-shift depends on the frequency of the fundamental component of the switched mode voltage. Due to the frequency dependent phase-shift, the control signals of the primary switches are not directly applicable for controlling the rectification switches of the secondary side of the resonant converter. Resonant converters provide, however, significant advantages because zero voltage switching “ZVS” conditions or zero current switching “ZCS” conditions can be arranged for the primary and/or secondary switches, and thus the switching losses can be reduced.
Publication U.S. Pat. No. 7,184,280 describes a method for taking the above-mentioned frequency dependent phase-shift into account when generating control signals for the rectification switches of the secondary side of a resonant converter. The method relies, however, on having a sufficiently accurate model of the resonant converter. Inaccuracies between the model and the physical device lead to increased safety margins that in turn lead to increased losses in the rectification switches.
Another principle for controlling rectification switches is based on measuring currents of the rectification switches. For example, when a MOSFET is used for mimicking a diode, the MOSFET can be driven on when current starts flowing through its parasitic diode and driven off when the current stops. In conjunction with some commercial circuits, the current measurement is based on the voltage-drop over a current conducting rectification switch. This method is however not very robust, because it involves measuring millivolt level signals in an environment that can be very noisy like in a case of an SMPS. The current can also be measured using a current transformer. The current to be measured flows in the primary winding of the current transformer, and the current of the secondary winding of the current transformer can be rectified and transformed into a voltage signal using a shunt resistor, or reverse-parallel connected diodes, or some other suitable electrical entity comprising one or more electrical components. With suitable design of the current transformer, the voltage level can be in the range of volts instead of millivolts, which makes robust measurement much more feasible. In order to detect a situation where the current is flowing and also a situation where the current is off, the signal indicative of the measured current is typically compared with a threshold value. The rectification switch is controlled to be in the conductive state when the signal reaches the threshold value, whereas the rectification switch is not controlled to be in the conductive state when the signal is below the threshold value. In conjunction with an SMPS, the above-described principle is however not free from challenges. One of the challenges is related to the tendency to oscillatory behavior when loading is such that the current is in the vicinity of its threshold value. When the current drops below the threshold value, the rectification switch is no longer controlled to be conductive and the current flows via the parasitic diode of the rectification switch. As a corollary, the voltage-drop over the rectification switch increases. This causes that the output voltage of the SMPS drops and the control of the SMPS takes control actions to increase the output voltage. As a corollary of the control actions, the current increases and it may exceed the threshold value. This causes that the rectification switch is controlled to be conductive and thus its voltage-drop decreases. As a corollary, the output voltage of the SMPS increases, and the control of the SMPS takes control actions to decrease the output voltage. This may cause in turn that the current drops again below the threshold value. The above-described chain of actions can repeat itself and thus the oscillatory behavior takes place. The threshold value can be varied based on the load. This can alleviate the above-described issue, but adds complexity to the control circuit.